Tunable lenses are optical elements whose optical characteristics, such as the focal length and/or the location of the optical axis, can be adjusted during use, typically under electronic control. Such lenses may be used in a wide variety of applications. For example, U.S. Pat. No. 7,475,985, whose disclosure is incorporated herein by reference, describes the use of an electro-active lens for the purpose of vision correction.
Electrically-tunable lenses typically contain a thin layer of a suitable electro-optical material, i.e., a material whose local effective index of refraction changes as a function of the voltage applied across the material. An electrode or array of electrodes is used to apply the desired voltages in order to locally adjust the refractive index to the desired value. Liquid crystals are the electro-optical material that is most commonly used for this purpose (wherein the applied voltage rotates the molecules, which changes the axis of birefringence and thus changes the effective refractive index), but other materials, such as polymer gels, with similar electro-optical properties can alternatively be used for this purpose.
Various designs for lenses based on liquid crystals are known in the art, although their practical uses have been limited. For example, Naumov et al. describe one approach for this purpose in “Liquid-Crystal Adaptive Lenses with Modal Control,” Optics Letters 23, pages 992-994 (1998), which is incorporated herein by reference. The authors used nematic liquid-crystal (LC) phase correctors to form spherical and cylindrical wave fronts. A LC cell with a distributed reactive electrical impedance was driven by an AC voltage applied to the cell boundary to yield the desired spatial distribution of the refractive index.
Other tunable lens designs use an electrode array to define a grid of pixels in the liquid crystal, similar to the sort of pixel grid used in liquid-crystal displays. The refractive indices of the individual pixels may be electrically controlled to give a desired phase modulation profile. (The term “phase modulation profile” is used in the present description and in the claims to mean the distribution of the local phase shifts that are applied to light passing through the layer as the result of the locally-variable effective refractive index over the area of the electro-optical layer of the tunable lens.) Lenses using grid arrays of this sort are described, for example, in the above-mentioned U.S. Pat. No. 7,475,985.